Interference can degrade the performance of communication systems. The interference signal can be a signal source unrelated to the communications system, such as the signal from another transmitter or it can be multipath. Multipath occurs when the transmitted signal of interest arrives at the receiver simultaneously from more than one direction due to reflections from buildings, hills, etc. An adaptive array is a good means of rejecting interference signals.
An adaptive array with N antenna elements can reject N-1 interference signals. There is cost, space and esthetic advantages to be able to reduce by a factor of 2 the number of antennas that an adaptive array requires to reject interference signals. The space and esthetic advantages are particularly true when a single antenna replaces the two conventional antennas of a two input adaptive array.
Adaptive array theory can be found in Bernard Widrow, Proceedings of the IEEE, Vol.55, No.12, December 1967, p.2143; Robert A. Monzingo and Thomas W. Miller, Introduction to Adaptive Arrays, John Wiley & Sons, New York, 1980; and Bernard Widrow and Samuel D. Stearns, Adaptive Signal Processing, Prentice-Hall, 1985.
The antenna elements can be omni-directional or of other antenna types. Theory for the different antenna types can be found in texts such as Kai Fong Lee, Pronciples of Antenna Theory, John Wiley & Sons, New York, 1984: Ronald W.P. King, The Theory of Linear Antennas, Harvard University Press, Cambridge, MA, 1956; and Thomas Milligan, Modern Antenna Design, McGraw-Hill Book Company, New York, 1985.
The type antenna used in an application is generally dictated by the application requirements: desired antenna pattern, gain, cost, space, esthetics, convenience, convention, etc. For example, the rabbit ears antenna used in TV receivers, which is a form of dipole antenna, is simple, inexpensive, portable, space saving, directional, and less esthetically objectionable than most other types of indoor antennas. Like other applications, it does not use direction of arrival information; the rabbit ears is generally connected to the TV in a manner that does not preserve the direction of arrival information, via a single transmission line. Theoretically, the rabbit ears and the center fed dipole antenna are viewed as an extension of a single transmission line.
In the adaptive array, which requires direction of arrival information, the type of antennas that are used is determined by the application. For example, in rejecting multiple jamming signals in a military communications receiver, a rabbit ears/dipole antenna is not appropriate. Although the dipole and other two terminal antennas have been used with adaptive arrays, they have been used as single antenna elements as part of arrays. Used in this conventional manner, the adaptive array requires N two terminal antennas, each acting as a single element, to reject N-1 interference signals.
In conventional transmission line theory, such as presented by Pierre Grivet, The Physics of Transmission Lines at High and Very High Frequencies, 1970, Academic Press, and Bharathi Bhat, Analysis, Design, and Application of Transmission Lines, 1987, Artech House, Norwood, MA., a single transmission line is connected to the dipole or two terminal antenna. But this is adequate only when the diphole or two terminal antenna is used as a single element and the complete signal from the antenna is not required.